1. Field of the Invention
The present invention relates to distributed antenna systems, communication control methods, and base station apparatuses.
2. Description of the Related Art
Distributed antenna systems (DASs) in which antennas are distributed and disposed in a building have been known as related art in the technical field of the present invention. In the conventional distributed antenna systems, a method has been known in which a leakage coaxial cable is laid from a base station apparatus and electromagnetic waves are emitted around the laid cable. A method has also been known in which an analog transfer signal input to or output from a base station apparatus is distributed to a plurality of antennas with coaxial cables through an apparatus that branches or couples the signal. In the conventional distributed antenna systems, since a signal input to or output from a base station apparatus is distributed to a plurality of antennas, an identical signal is input to or output from all the antennas.
In recent years, in high-speed radio communication methods such as long term evolution (LTE) and worldwide interoperability for microwave access (WiMAX), the multiple input multiple output (MIMO) technology, in which data is sent from a plurality of antennas and received by a plurality of antennas, has been employed in terms of the improvement of frequency use efficiency. In this MIMO technology, since different signals need to be sent from and received by a plurality of antennas, for example, it is demanded also in distributed antenna systems that different signals be sent from and received by a plurality of antennas.
Applying MIMO to the above described DASs can increase the system capacity.
Related art used when the MIMO technology is applied to a distributed antenna system is described in Japanese Unexamined Patent Application Publication No. 2010-068496 (patent document 1). This document discloses that a power measurement section of a terminal measures the receiving power of a pilot signal sent from each antenna of a distributed antenna system for a long period; the terminal selects a predetermined number of antennas having high receiving power as communication antenna candidates, and reports the communication antenna candidates and the corresponding receiving power to a radio base station apparatus; a channel estimate section of the terminal receives communication possible antennas and antenna index information assigned thereto from the radio base station apparatus, and performs channel estimation for the communication possible antennas; and, since it is necessary to determine a precoding matrix to be used in calculation applied to a transmission signal from the base station apparatus, in order to perform MIMO communication, precoding matrix indexes (PMIs) related to the communication possible antennas are obtained according to the channel estimation in the terminal, and the PMIs are reported to the radio base station apparatus in communication by using the antenna index information, so that the phase rotation, power ratio, and other factors of each antenna used to perform data communication are controlled.